The invention relates to a method of measuring the phase difference of two periodic signals of the same frequency.
The phase difference F between two signals U and V of the same frequency is often measured by counting the cycles of a timing signal of a much higher frequency between successive zero passages of signals U and V. There is normally a synchronicity relationship between the frequencies of the timing signals t and signals U and V, so that in each full cycle of e.g. U, there is a constant, known number of timing pulses. This synchronicity is generally produced by means of phase-locked loops (PLL).
If, as a result of noise or random phase jitter, a single phase measurement does not provide the necessary accuracy, averaging takes place over several elementary measurements in successive, but not necessarily immediately successive cycles P of U. As the value for the phase difference F is unambiguous only in the range O.ltoreq.F&lt;P, such averaging may provide erroneous results when F lies near a boundary of said range. This problem is usually referred to as 2.pi. ambiguity.
Already known means to remove this uncertainty require considerable circuit expenditure, particularly with regard to analog circuits.
The PLL is in part an analog assembly. If its frequency, which corresponds to a clock frequency, is modulated with the signal frequency U or V, systematic non-linearities may occur in the phase measurement. These non-linearities do not disappear when the measurement is averaged over several elementary measurements. Filtering may remove the non-linearities. However, the filter forming part of the PLL must be a low-pass filter with a very low limit frequency, so that the PLL is slow. It is therefore difficult in many cases to find a satisfactory compromise between linearity and control rate.
The aim of the present invention is to improve the measuring method by reducing to a minimum the constructional expenditure necessary for performing the method, particularly with regard to costly analog circuits.
As a result of the present invention, the aforementioned difficulties in measurement are obviated by using an asynchronously operating clock generator, such as a crystal oscillator.